Yup, encoded messages. The echo is multipath reception, sometimes a signal will take both short path and long path at the same time resulting in the long one delayed enough to echo. Sometimes a signal will travel around the world more than once resulting in multiple path delays, more echoes. BTW, no such thing as ground wave above about 3mHz, somebody was a bit off on that one unless you're very close to the transmitter and receiving direct wave. There is a difference, ground wave follows the Earth-sky boundary over long distances while direct wave propagates solely in the atmosphere over short distances.
There is yet another possibility, two or more transmitters in different parts of the world operating on the same frequency simultaneously. With the military comms nets being what they are it just could be the transmissions are global in nature and such a system ensures complete coverage at all times. The USCG uses such a linked system on VHF FM marine frequencies and sometimes with all transmitters going at once those in between get heterodynes making reception difficult to impossible. The logic rather escapes me, better coverage along the coast with man made holes in it? DOH!
No argument from the capture ratio peanut gallery, what works on paper seldom does in practice. I used to live about halfway between Atlantic City and Barnegat and heard the mess every time an alert was broadcast.
